The invention concerns a device to de-water fiber suspensions as used for instance in the fabrication of cardboard. The device consists of a perforated drum, the wall of which contains a multitude of openended radial cells, a drive which turns this drum in a given direction, a material feeder, an installation to facilitate the tangential removal of the product, namely a fibersheet, and a trough to catch and remove the suspended water originating from the fiber suspension in the above mentioned perforated drum.
Commonly the de-watering of fiber suspension in the filter drums is achieved by using negative pressure, e.g. vacuum, to direct the water towards the center of the drum and to drain it from there toward the outside. The vacuum system needed for this process is costly in installation and use, and uneveness or flaws in the fibersheet cannot be prevented. The "Scott Former" built by the American firm Black-Clawson uses a different principal. They use a perforated vacuum roller with a bronze sieve. The lower edge of the fibersheet-runoff runs along the filter drum's vertex while the upper edge protrudes and thereby forms the fibersheet under pressure. The extracted water is directed towards the filter drum's center by vacuum and then expelled by centrifugal force behind the fibersheet forming zone. A device in accordance with this principle of using a vacuum roller, is the recently better known "Honeycomb Roller" described in U.S. Pat. Nos. 3,100,928 and 3,590,453. The Honeycomb Roller is by merit of its cell structure superior to the more conventional vacuum rollers. The Honeycomb uses a maximum effective area, is relatively light in weight, thereby simplifying installation and, resulting in less stress on the bearings, and it shows high stability and little divergence from true alignment. These features make a high strain factor possible. The large size cells reduce the danger of plugging up since the inside is open. The installation of vacuum chambers and other options is relatively simple; furthermore, by using the Honeycomb a nontouching and therefore wearproof dynamic vacuum chamber seal installation is possible. By moving the endseals during the operation, the format of the product can even be changed.
Yet, at high production operation at high speed, it became evident that the centrifugal water extraction immediately behind the forming zone caused damage to the freshly formed fibersheet at or near the expulsion area and proved to be less satisfying.
The purpose of the invention is to provide a de-watering system as defined above; namely a system where the extracted water will be removed from the roller, but instead of by centrifugal force immediately behind the forming zone in a radial direction, it will be removed at a safe distance from the forming zone in a tangential direction and away from the fibersheet. The formation of an even, fine and vertical film of water is being striven for by the invention.
Furthermore, it is necessary to prevent the disorientation of the individual fibers during the forming process by outside influences or by the structural character of the roller surface. To achieve this, the negative pressure or vacuum on the inside of the perforated drum underneath the forming zone should be kept at a minimum, and a honeycomb structure of the cells should be avoided.